This invention relates to the field of polymeric membranes, and provides a membrane useful for removing water or water vapor from air or other gas streams.
Compressed air is commonly used to drive pneumatically operated equipment, such as air-driven motors for hand tools, pneumatically operated valves, and the like. One problem arising from the use of compressed air is that, because there is usually water vapor in the air before the compression step, the process of compression produces liquid water. The result is that liquid water becomes entrained with the compressed air. If the water is not removed, it is likely to cause rusting of components, and it may impair the performance of the motor, valve or other component being operated. Moreover, in the case of a pneumatically operated valve, the water may clog the valve.
Compressed gases have other uses, such as in the preservation of perishable materials. In such cases, it is important to avoid condensation which may lead to mildew or mold, in addition to causing corrosion of the equipment.
One prior art solution to the above-described problem is to use a cryogenic system, which refrigerates the air so as to liquefy as much water as possible. The water is preferably cooled to a temperature that is higher than the dewpoint of the air. The air is therefore in a superheated condition, wherein it has a dewpoint slightly above freezing, so that as long as the air temperature remains above the dewpoint, no liquid water will form. A cryogenic system has the obvious disadvantage that it requires the transportation and storage of cryogenic liquids, which inherently consumes energy. Also, the cryogenic liquids must be periodically replenished.
Another solution proposed in the prior art has been the use of a molecular sieve, i.e. a packed bed that absorbs water. But a packed bed has the disadvantages that it is not feasible for use in a continuous process, and that it must be periodically regenerated or replaced when its pores have become filled.
Polymeric membranes have long been used in the non-cryogenic separation of components of air. Such membranes have also been used in separating water from air. A membrane has the inherent advantage that it does not require energy to operate. A membrane system has no moving parts, and thus requires little or no maintenance. However, the membrane dryers of the prior art have been less efficient than other drying methods of the prior art. In general, prior art membranes have shown an efficiency of about 75%, compared with efficiencies of up to about 98% with some prior art techniques.
The present invention provides a membrane, and a method of making the membrane, having substantially improved efficiency in removing water from compressed air.